C  

Fluoride rapidly complexes with divalent cations; giving a calcium-rich liquid (e.g., milk) in the stomach binds the ingested fluoride to insoluble CaF₂, limiting further absorption and systemic toxicity. Guidelines for acute fluoride ingestion therefore recommend immediate oral calcium (milk, Ca-containing antacid) before transfer for definitive care, whereas plain water or bicarbonate do not chelate fluoride and delay increases risk [Whitford, 2011, PMID 21701192; McIvor, 1990, PMID 2182050].